The Metropolitan Police says two police officers have been taken to hospital after being stabbed in central London.
The stabbing took place near Leicester Square at 06:00 BST and is not being treated as terror-related although the injuries are not life-threatening.
London Mayor Sadiq Khan branded the attack “utterly appalling” at a “momentous time for our country”.
A man has been arrested on suspicion of grievous bodily harm and assaulting an emergency worker.
A Taser was deployed on Great Windmill Street, near Shaftesbury Avenue, and the man is also being treated in hospital.
The stabbing took place close to Haymarket, between Leicester Square and Piccadilly Circus, which is home to many of London’s theatres and Chinatown.
The male officers’ families have been told, Scotland Yard said.
During the night, two other officers were injured, including one who was driven into by a car in south London, according to London’s mayor.
Mr Khan said: “These brave officers were doing their duty and assisting the public at this momentous time for our country.
“My thoughts and prayers are with them, their loved ones and police colleagues following this disgraceful attack.”
He also told the BBC “neither officer is in a life threatening condition”.
Martin Hewitt, the chair of police chiefs, described the attack as “terrible news”.
BBC